I doubt Canon can change much without affecting their own lenses - Some of my EOS lens dated back to more than a decade and new lens technology have changed a lot since - Whatever they do it must be backward compatible with my old 85mm f/1.2

Im hoping this is not related to a canon V sigma problem as I intend buying a sigma long lens for my husband to use on my old 300D

It is NOT related.

I routinely use a mixture of Canon, Sigma, and Tokina Canon-mount lenses on my Canon dSLRs. As long as you are not purchasing prehistoric lenses (read pre-digital) where some problems can and do crop up, you will be fine.

Sigma makes very good competitive lenses which sell at very competitive prices. Your 300D will do well as long as you purchase the Sigma in the Canon EOS mount. The brand issue (and loyalty) is similar to that of Ford, Chevy, Caddy, Chrysler, etc. — personal preference and personal belief. (Hey, I prefer my Zeiss lenses to the other brands, but I still successfully use 'em!)